This invention relates generally to the manufacture of print wheels for impact printers of the type which include elastically deformable, concentrically arranged radial arms having type font characters at their free ends and a plastic molded hub having a central passage.
In particular, the invention provides a method of molding the aforementioned hub to provide a precise diameter central passage without requiring further machining of the molded part subsequent formation.
Print wheels of the type concerned are required to be perfectly plane, to have their radial character carrying arms concentric and coplanar, at their free ends and have a central mounting passage of precise diameter with maintenance of precise tolerance specifications. Preferably, at least the hub portion of the print wheel is molded radially outwardly from the center thereof to assure at least initial concentricity and parallel coplanar character disposition. In practice, difficulties are encountered when molding in a radially outward direction in attaining and maintaining the desired concentricity and the precision tolerances demanded of the central passage.
In addition to requiring precision machining of the molded article subsequent to molding and adding thereby to the cost of the article; there is inherent shrinkage, deformation, etc., so that even if the articles are machined precisely once formed, the resultant articles may well change and the precision obtained in the resultant articles may be changed. Even relatively the tolerances required to provide a useful, effective product are difficult to obtain and maintain on a practical and/or economic basis. Minor variations are significantly effective to sharply limit the life of the print wheel and its utility.
Reliance upon the machining operation subsequent to formation of the hub effectively force the concentricity and diameter tolerances of the central passage to be dependent upon the precision of the machining operation. There are too many variable to justify such reliance.
In addition, machining of the completed hub to achieve concentricity and meet the precise diameter specification requires breaking or cutting of the surface of skin of the plastic molded article which is formed during molding. Ordinarily, such plastic "skin" affords resistance to the effects of humidity. Porosity of the finished product is increased with the resultant increased deleterious effect upon the absorbancy of environmentally present materials including some fluids, gases, solvents, etc. Thus, even if one could initially achieve desired precision and obtain concentricalty by machining, distortion due to the resulting increase in porosity of the article molded such as shrinkage, absorption of fluids, etc. would materially reduce the chances of maintaining the precision and concentrically within desired fine tolerance over the desired useful life of the article.
It would be desirable to form the hub, etc., whereby the precision of the central passage of the hub is independent totally of the molding conditions.
Further, opposite axial surfaces of the hub should be parallel to assure the proper disposition of the print wheel and of the characters carried thereby during use.
The printing wheel ordinarily is mounted on one of its axial faces. If the opposite axial sides of the hub of the print wheel were not perfectly planar and perpendicular to the axis of the central passage, the printing wheel properly could not be mounted. Repair of such defects would involve machining of the opposite surfaces to dress the lateral faces. Again, the process involved is expensive. As with any machining involving cutting, one would break the surface film or skin of the plastic material which is formed at the time of molding. This would impart increased porosity to the material. The useful life of the article materially would be reduced. The deleterious effect of humidity, etc., would be a material factor in reducing its useful life, causing unacceptable distortion and change exceeding tolerance specifications.